1. Field of the Invention
The present invention relates to a battery voltage detection circuit.
2. Description of the Related Art
Devices such as a notebook computer using rechargeable batteries must accurately detect voltages of the batteries to control charging/discharging of the batteries connected in series. FIG. 7 depicts a typical configuration of a battery voltage detection circuit (see Japanese Patent Application Laid-Open Publication No. 2002-243771). A battery voltage detection circuit 100 is a circuit for detecting voltages of four batteries BV1 to BV4 connected in series and includes an operational amplifier 110, resistors R1 to R4, switches SW0M to SW4M, SW0P to SW3P, and a power source 115 that outputs a reference voltage VREF. When detecting a voltage VBV4 of the battery BV4 in such battery voltage detection circuit 100, the switches SW4M and SW3P are turned on and other switches are turned off. As a result, a voltage VOUT is output from the operational amplifier 110 to an AD converter (ADC) 120 according to a difference between a voltage V4 of the positive side terminal of the battery BV4 and a voltage V3 of the negative side terminal thereof. The voltage VBV4 of the battery BV4 may be detected by converting the voltage VOUT into a digital value with the ADC 120. Similarly, a voltage VBV3 of the battery BV3 may be detected by turning on the switches SW3M and SW2P and turning off other switches. A voltage VBV2 of the battery BV2 may be detected by turning on the switches SW2M and SW1P and turning off other switches. A voltage VBV1 of the battery BV1 may be detected by turning on the switches SW1M and SW0P and turning off other switches.
When lithium-ion batteries are used for the batteries BV1 to BV4, each of the voltages VBV1 to VBV4 across their respective batteries BV1 to BV4 reaches nearly 4.5 V when fully charged. If it is assumed that the voltages VBV1 to VBV4 of the batteries BV1 to BV4 are set to 5 V in consideration of allowance in design, the batteries BV1 to BV4 connected in series generate a total voltage of 20 V, and thus, the battery voltage detection circuit 100 needs high voltage endurance. On the other hand, circuits for control systems including the ADC 120 typically use a power supply voltage of about 3.3 V, and a voltage VOUT output from the battery voltage detection circuit 100 must be 3.3 V or less.
If the resistors R3 and R4 have resistance values of R3 and R4, respectively, a gain GAMP of the operational amplifier 110 is expressed by R4/R3. Therefore, the voltage VOUT output when detecting the voltage VBV4 of the battery BV4 is expressed by VOUT=VBV4GAMP+VREF=(V4−V3)R4/3+VREF. Assuming that VBV4 is 5 V and VREF is 0.2 V, a condition of the gain GAMP of the operational amplifier 110 for achieving VOUT≦3.3 V is expressed by GAMP≦(VOUT−VREF)/VBV4=(3.3−0.2)/5≈0.6. Therefore, the voltage VOUT output to the ADC 120 may be set to a voltage of 3.3 V or less by selecting the resistance values of the resistors R3 and R4 such that the gain GAMP of the operational amplifier 110 is set to about 0.6. However, the operational amplifier 110 needs the high voltage endurance in this case, and thus, the costs of the battery voltage detection circuit 100 is increased.
To eliminate the need for the high voltage endurance in the operational amplifier 110, a voltage applied to the operational amplifier 110 is required to be 3.3 V or less. That is, to set a voltage V+ applied to a positive input terminal of the operational amplifier 110 to 3.3 V or less, (V3−VREF)R4/(R3+R4)+VREF≦3.3 is required to be satisfied. This leads to R4/(R3+R4)≦(3.3−VREF)/(V3−VREF)=(3.3−0.2)/(15−0.2)=3.1/14.8≈0.21. Therefore, the gain GAMP of the operational amplifier 110 is GAMP=R4/R3≦0.21/(1−0.21)≈0.26. Therefore, the need for the high voltage endurance in the operational amplifier 110 may be eliminated by selecting the resistance values of the resistors R3 and R4 such that the gain GAMP of the operational amplifier 110 is set to about 0.26. However, since the gain GAMP of the operational amplifier 110 is small in this case, the voltage VOUT input to the ADC 120 is reduced. Therefore, the highly-accurate ADC is necessary to accurately detect the battery voltages, resulting in increase in costs.
Furthermore, in the battery voltage detection circuit 100, a current is passed through the resistors R1 and R3 respectively connected to the input terminals of the operational amplifier when detecting the voltages of the batteries BV1 to BV4. Therefore, in order to restrain discharge from the batteries BV1 to BV4 due to this current, the resistors R1 and R3 needs large resistances of the order of a few megohms. In order to accurately detect the voltages of the batteries BV1 to BV4, the resistors R1 to R4 are required to have less voltage dependency in resistance values. Thus, when producing an integrated circuit including such resistors having large resistance values and less voltage dependency, a special processing is necessary, resulting in increase in costs.